But the theory was largely abandoned after an 1887 experiment by physicists Albert Michelson and Edward Morley.

Dubbed "the most famous failed experiment," the test was meant to gather data on the effects of this so-called ether wind. But the experiment showed that light propagation was not affected, suggesting ether wind did not exist.

Later, Einstein based his theory of special relativity on the idea that light can move through an ether-free vacuum.

Starkman's conception of ether, however, is very different from the outmoded 19th-century onehe thinks that ether affects the pull of gravity, not the movement of light waves.

"With traditional gravitational models, you have a rubber sheet that curves wherever there's a large mass on it," he said.

In Starkman's theory of how ether works, "when ether is around, the rubber sheet gets softer. So when you put a large mass on the sheet, the effect of the mass goes out further."

Starkman's initial calculations show that ether's localized effects on gravity would account for the high velocities of galactic stars.

The next phase in his research will be to perform more detailed calculations to make sure his ether theory matches up with empirical evidence, such as the motion of planets within the solar system.

"It's important to do these experiments, because either we'll be able to rule dark matter out or we'll increase our confidence in it.

"At this point I don't think we can rule out either of the two [competing] theories," he said.

Challenging Einstein

Several high-profile theoretical physicists have lined up to support Starkman's theory, including Jacob Bekenstein, theoretical physics professor at Hebrew University in Jerusalem, Israel, and Andreas Albrecht, cosmologist and physics professor at the University of California, Davis.

Still, Starkman acknowledges that his theory is in its infancy and may not stand up to rigorous testing.

"We're offering an alternative to the dark matter theorywe're not saying it's wrong. If I had to bet today on which of these theories was correct, I might bet on dark matter."

Meanwhile, many other experts are sitting on the fence.

Michael Turner, an astrophysicist at the University of Chicago in Illinois, is intrigued by Starkman's theory, but he hesitates to accept it wholesale due to its troubling implications.

For example, the presence of ether would create holes in Einstein's theories of relativity, the widely accepted explanations for how light moves and gravity works (read an excerpt and see images from "Einstein and Beyond" in National Geographic magazine).

"It's early to tell whether this [ether] theory will really pass through the gate," Turner said. "When you change the theory of gravity, you could cause lots of problems elsewhere.

"It's an interesting Plan B, but we already have a pretty good Plan A."